This invention relates to lasers and more particularly to conductively cooled optically pumped solid state lasers.
Laser rods and pump lamps of solid state lasers typically are cooled by liquids or gases flowing over their outer surfaces. Such laser assemblies are inherently complex, expensive and often have design limitations for a number of reasons. For example, the coolant often decomposes under the intense ultraviolet radiation generated by the flashlamp itself, and this limits the life of the laser unless the coolant is changed frequently to maintain strict purity requirements. Also the mechanical circulating system components must be regularly maintained to prevent a system failure. In addition, circulating fluid cooling systems usually require seals which complicate and increase the cost of construction, installation and maintenance of the laser assembly.
Other designs of solid state lasers have used conduction cooling by connecting laser rods and lamps directly to the heat sink. This design is often used where the laser is meant to operate at temperatures that are above ambient room temperatures. To establish thermal contact between the heat source (rod or lamp) and the heat sink, the two are permanently affixed by brazing, bonding or soldering with compounds that generally exhibit limited light reflectivity. Such technique is described, for example, in U.S. Pat. No. 4,170,763. The problem with brazing, bonding or soldering the heat sources to the sink is that over time the brazing, bonding or soldering compounds may fail to maintain complete adhesion over the surfaces being joined, producing gaps between the heating elements and the heat sink. Such gaps reduce the thermal conductivity where ever they occur. The bonding or soldering materials fail over time, as do the coolants, either due to the intense radiation from the laser or because of the temperature cycling and the accompaning expansion/contraction of the materials. Unless the thermal coefficients of expansion are closely matched, the thermal stresses will eventually cause material fatigue and the unwanted gaps. Another major disadvantage of brazing, bonding, or soldering is that, unless thermal expansion coefficients of the adhesing material and both surfaces being joined are closely matched, large stresses may be developed in the laser rod. These stresses are often sufficient to produce either significant birefringence lasses or to strain the laser rod to the material fracture limit.
Another technique for establishing such thermal contact is pressing the heat source into, or otherwise surrounding it with a thermally-conductive medium such as a dry dielectric powder which provides heat transfer to a large heat sink. The problem with the foregoing conductively cooled systems is that the individual components are not easily optimized. Materials having acceptable thermal and reflective properties cannot readily be optimized for both. Furthermore such materials cannot readily be brought into intimate contact with the heat/light sources over large cylindrical surface areas as required for efficient pumping and heat removal. In addition, the thermal expansion of the rod or pump will cause the powder to eventually compact. This produces, as was the problem with solders, gaps between the rod or pump and the conductive powder. However, in this case both the thermal conductivity and the reflectivity of the assembly degrades.
This invention is directed to a laser construction which overcomes these difficulties.